a) Field of the Invention
The present invention relates to a vari-focal lens system having at least one graded refractive index lens.
b) Description of the Prior Art
In the recent years where electrical and mechanical systems have been made more compact in silver salt cameras and video cameras, lens systems, especially vari-focal lens systems, for such cameras have been demanded that satisfy the requirements of compact designs, light weights and high specifications. Under such circumstances, it is very difficult to correct aberrations favorably in lens systems only using homogenous spherical lenses and aspherical lenses are used for correcting aberrations. However, the above-mentioned requirements cannot be satisfied even by using aspherical lenses.
For this reason, attention is now attracted to lens systems having graded refractive index lenses (GRIN lenses).
As the conventional zoom lens systems using GRIN lenses for lens shutter cameras, there are known, for example, the lens systems disclosed by Japanese Patent Kokai Publication No. Sho 61-148414 and Japanese Patent Kokai Publication No. Sho 61-295524. These zoom lens systems are designed so as to have total lengths shortened by arranging negative lens units on the extreme image side. However, these zoom lens systems cannot provide sufficient vari-focal effects due to low vari-focal ratios of 1.5 to 2 and total lengths of these lens systems are not short for the vari-focal ratios thereof.
Further, as lens systems which are suited for use in single reflex lens cameras or electronic still cameras and comprise GRIN lenses, there are known the lens systems disclosed by Japanese Patent Kokai Publication No. Sho 61-126515 and Japanese Patent Kokai Publication No. Sho 61-138913.
Refractive indices of the GRIN lenses manufactured by the ion exchange method, Sol-Gel method, etc. are distributed, on the n.sub.d -.nu..sub.d graph, from the high refractive index-high dispersion region to the low refractive index-low dispersion region, and it is deemed that GRIN lenses having such refractive index distributions are manufacturable rather easily in practice. In contrast, both the GRIN lenses used in the above-mentioned lens systems have refractive indices distributed from the high refractive index-low dispersion region to the low refractive index-high dispersion region, and it cannot be said that these GRIN lenses are used effectively from the viewpoint of practical manufacturing of the GRIN lenses.